Fiber optic strain sensors are used in applications such as smart structures and health monitoring. The advantages of these sensors include their small size, low cost, multiplexing capabilities, immunity to electromagnetic interference, intrinsic safety and their capability to be embedded into structures.
Many structural devices and objects undergo various shape changes when exposed to certain environments. In some instances, it is necessary to know the degree of change and to compensate for these changes. By embedding or attaching a sensor to the structure, one can monitor the dynamic shape or relative position of the structure independently from temperature or load effects. Further by measuring the dynamic shape of a structure, the state of flexible structures can be established. When a degradation of the measured signal occurs, it can be corrected using signal processing.
Some have tried to measure shape changes by using foil strain gauges. These sensors, while sufficient for making local bend measurements, are impractical for use with sufficient spatial resolution to reconstruct shape or relative position over all but the smallest of distances. Others have used fiber optic micro-bend sensors to measure shape. This approach relies on losses in the optical fiber which cannot be controlled in real-world applications.
Clements (U.S. Pat. No. 6,888,623 B2) describes a fiber optic sensor for 3-D position measurement. The central system component of the invention is a flexible “smart cable” which enables measurement of local curvature and torsion along its length. These quantities are used to infer the position and attitude of one end of the cable relative to the other. Measurements of the local curvature and torsion along the cable allow reconstruction of the entire cable shape, including the relative position and orientation of the end points. The smart cable for making these measurements comprises a multicore optical fiber, with individual fiber cores constructed to operate in the single mode regime, but positioned close enough to cause cross-talk (mode coupling) between cores over the length of the fiber. This cross-talk is very sensitive to the distribution of strain (curvature and torsion) along the cable. Clements describes the errors in measured curvature as being divided into three classes: those due to instrument noise, systematic errors due to fabrication defects (core geometry, index of refraction variations, etc.), and sensitivity to extrinsic variables such as temperature. Of the three, instrument noise is probably the worst threat to successful shape inversion. Several approaches are proposed to mitigating effects of instrument noise, including time averaging and diversity measurements using fibers with redundant cores or multiple multicore fibers. A plurality of single mode cores may also be provided in an optical medium comprising a flexible sheet of material.
Greenaway et al. (U.S. Pat. No. 6,301,420 B1) describe a multicore optical fiber for transmitting radiation. The optical fiber comprises two or more core regions, each core region comprising a substantially transparent core material and having a core refractive index, a core length, and a core diameter. The core regions are arranged within a cladding region. The cladding region comprises a length of first substantially transparent cladding material having a first refractive index. The first substantially transparent cladding material has an array of lengths of a second cladding material embedded along its length. The second cladding material has a second refractive index which is less than the first refractive index, such that radiation input to the fiber propagates along at least one of the core regions. The cladding region and the core regions may be arranged such that radiation input to the optical fiber propagates along one or more of the lengths of the core regions in a single mode of propagation. The optical fiber may be used as a bend sensor, a spectral filter or a directional coupler. A bend sensor comprises a multicore photonic crystal fiber. The measurement of the relative shift in the fringe pattern provides an indication of the extent by which the fiber is bent. If the fiber is embedded in a structure, an indication of the extent to which the structure is bent is provided. This type of system is an intensity based system, in contrast to an internal reflection system, therefore not all light is guided by an internal reflection mode and, hence, the system is not as accurate as an internal reflection system.
Greenway et al. (U.S. Pat. No. 6,389,187 B1) describe an optical fiber bend sensor that measures the degree and orientation of bending present in a sensor length portion of a fiber assembly. Within a multicored fiber, cores are grouped in non-coplanar pairs. An arrangement of optical elements define within each core pair two optical paths which differ along the sensor length. One core of a pair is included in the first path and the other core in the second path. A general bending of the sensor region will lengthen one core with respect to the other. Interrogation of this length differential by means of interferometry generates interferograms from which the degree of bending in the plane of the core pair is extracted. Bend orientation can be deduced from data extracted from multiple core pairs. The apparatus is capable of determining bending of the sensor length, perhaps as a consequence of strain within an embedding structure, by monitoring that component of the bend in the plane of two fiber cores within the sensor length. Interferograms are formed between radiation propagating along two different optical paths, the optical paths differing within a specific region of the fiber. This region, the sensor length, may be only a fraction of the total fiber length. Generally, bending this sensing region will inevitably lengthen one core with respect to the other. Interrogation of this length differential by means of interferometry provides an accurate tool with which to measure bending. Moreover, defining a sensor length down a potentially long fiber downlead enables strains to be detected at a localized region remote from the radiation input end of the fiber. Thus, the fiber assembly can be incorporated in, for example, a building wall, and strains developing in the deep interior of the wall measured.
The first and second cores constitute a core pair and component cores of the multicore fiber preferably comprise an arrangement of such core pairs. The coupling means may accordingly be arranged to couple and reflect a portion of radiation propagating in the first core into the second core of the respective pair. This provides the advantage of flexibility. The optical path difference arising between any core pair can be interrogated, enabling the selection of planes any of which may be the plane in which components of a general bend curvature may be measured.
Schiffner (U.S. Pat. No. 4,443,698) describes a sensing device having a multicore optical fiber as a sensing element. The sensing device includes a sensing element in the form of an optical fiber, a device for coupling light into the fiber and a device for measuring changes in the specific physical parameters of the light passing through the fiber to determine special physical influences applied to the fiber. The fiber is a multicore fiber having at least two adjacently extending cores surrounded by a common cladding and a means for measuring the alterations in the light passing through each of the cores. To make the device sensitive to bending and deformation in all directions, the fiber may have two cores and be twisted through 90 degrees or the fiber may have three or more cores which are not disposed in the same plane. The measuring of the amount of change may be by measuring the interference pattern from the superimposed beams of the output from the two cores or by measuring the intensity of each of the output beams separately. When there is no appreciable cross-coupling between the cores, an interferometric means for measurement will include a light receiving surface which is arranged in the path of light which passes through the two cores and has been brought into interference by means of superimposition. The sensing means may use a light receiving surface which is a collecting screen in which the interference pattern can be directly observed or the light receiving surface may be the light sensitive surface of a light sensitive detector which will monitor the light intensity of the interference pattern. To superimpose the light beams emitted from each of the cores, a beam divider device or devices may be utilized.
Haake (U.S. Pat. No. 5,563,967) describes a fiber optic sensor and associated sensing method including a multicore optical fiber having first and second optical cores adapted to transmit optical signals having first and second predetermined wavelengths, respectively, in a single spatial mode. The first and second optical cores each include respective Bragg gratings adapted to reflect optical signals having first and second predetermined wavelengths, respectively. Based upon the differences between the respective wavelengths of the optical signals reflected by the respective Bragg gratings and the first and second predetermined wavelengths, a predetermined physical phenomena to which the workpiece is subjected can be determined, independent of perturbations caused by other physical phenomena.
Froggatt and Moore, “Distributed Measurement of Static Strain in an Optical fiber with Multiple Bragg Gratings at Nominally Equal Wavelengths,” Applied Optics, Vol. 27, No. 10, Apr. 1, 1998 describe a demodulation system to measure static strain in an optical fiber using multiple, weak, fiber Bragg gratings (FBGs) in a single fiber. Kersey et al. in “Fiber Grating Sensors,” Journal of Lightwave Technology, Vol. 15, No. 8, August 1997 describe that a primary advantage of using FBGs for distributed sensing is that large numbers of sensors may be interrogated along a single fiber. With mixed WDM (wavelength division multiplexing)/TDM (time division multiplexing) in the serial configuration several wavelength-stepped arrays are concatenated, each at a greater distance along the fiber. Two deleterious effects can arise with strong reflectors. FBGs whose reflected light signals are separated in time, but which overlap in wavelength can experience cross-talk through “multiple-reflection” and “spectral-shadowing”. The WDM/TDM parallel and branching optical fiber network topologies eliminate these deleterious effects, but at the price of reduced overall optical efficiency and the need for additional couplers and stronger FBGs.
Froggatt (U.S. Pat. No. 5,798,521) describes an apparatus and method for measuring strain in Bragg gratings. Optical radiation is transmitted over a plurality of contiguous predetermined wavelength ranges into a reference optical fiber network and an optical fiber network under test to produce a plurality of reference interference fringes and measurement interference fringes, respectively. The reference and measurement fringes are detected and sampled such that each sampled value of the reference and measurement fringes is associated with a corresponding sample number. The wavelength change of the reference optical fiber, for each sample number, due to the wavelength of the optical radiation is determined. Each determined wavelength change is matched with a corresponding sampled value of each measurement fringe. Each sampled measurement fringe of each wavelength sweep is transformed into a spatial domain waveform. The spatial domain waveforms are summed to form a summation spatial domain waveform that is used to determine location of each grating with respect to a reference reflector. A portion of each spatial domain waveform that corresponds to a particular grating is determined and transformed into a corresponding frequency spectrum representation. The strain on the grating at each wavelength of optical radiation is determined by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement.
Chen et al. (U.S. Pat. No. 6,256,090 B1) describe a method and apparatus for determining the shape of a flexible body. The device uses Bragg grating sensor technology and time, spatial, and wavelength division multiplexing, to produce a plurality of strain measurements along one fiber path. Using a plurality of fibers, shape determination of the body and the tow cable can be made with minimal ambiguity. The use of wavelength division multiplexing has its limitations in that the ability to have precision with respect to determining the shape and/or position of an object is limited. Wavelength division multiplexing can only be used with sensor arrays that have less than one hundred sensors and, therefore, is insufficient for the application of determining shape and or position of an object with any precision.
An object is to provide a fiber optic position and/or shape sensing device that employs an optical fiber comprising at least two fiber cores disposed therein coupled with a frequency domain reflectomer.
Another is to provide a method for determining position and/or shape of an object using the fiber optic position and shape sensing device.